An optical fiber glass preform (hereinafter called a glass preform) is heated and melted by a heater etc. to thereby draw an optical fiber through a drawing furnace. Since the temperature of the inside of the drawing furnace becomes very high (2000° C. or higher), carbon is normally used in, for example, a furnace core tube for surrounding the glass preform. This carbon is oxidized and consumed in a high-temperature oxygen-containing atmosphere. In order to prevent this state, nitrogen gas or rare gas such as argon gas or helium gas (hereinafter called an inert gas etc.) is fed to the inside of the drawing furnace.
Also, normally, a diameter of the upper end of the glass preform is decreased in a taper shape and a dummy rod (also called a support rod) with a small diameter is connected and the glass preform is hung and supported inside the furnace core tube of the drawing furnace, but it is difficult to seal a joining portion of the dummy rod and the tapered portion with the diameter changing greatly, and it is difficult to fill the inside of the drawing furnace with the inert gas etc. As a result, there is a method in which an upper chamber is formed above the drawing furnace in a form of upwardly extending the furnace core tube and the glass preform including the joining portion of the dummy rod and the tapered portion is received inside the upper chamber and an outer peripheral surface of the dummy rod is sealed with the upper end of the upper chamber.
However, in this method, as drawing of the glass preform progresses and the glass preform is downwardly moved, capacity of space in the upper chamber is increased, with the result that pressure in the furnace varies and the flow of gas such as the inert gas fed to the inside of the furnace core tube changes with time and thereby, the amount of heat transfer of a melt part of the glass preform changes and the diameter of the glass fiber during the drawing varies.
On the other hand, for example, Patent Reference 1 discloses a method for forming a pipe with the same diameter as that of a glass preform above the glass preform and maintaining capacity of space in an upper chamber substantially constant even when drawing of the glass preform progresses.
FIG. 5A is a diagram showing an example of a drawing furnace disclosed in Patent Reference 1 described above. A glass preform 1 has a diameter contracted part 1c (also called a seed rod) in which a diameter of the upper end of a straight trunk part 1a (body part) is contracted through a taper part 1b, and is hung and supported by joining a dummy rod 2 (a shaft or a support rod) to the diameter contracted part 1c using, for example, a joining member 3. The outside of the dummy rod 2 is provided with a quartz pipe 10 having substantially the same outside diameter as an outside diameter of the straight trunk part 1a of the glass preform 1, and a pipe lower end 10a abuts on the peripheral edge of the taper part 1b of the glass preform 1.
A heating furnace is constructed by providing the outside of a furnace core tube 4 with a heater 5 and covering the outside of the heater 5 with a heat insulating material and surrounding the whole by a furnace cabinet 6, and an upper chamber 9 including a seal unit 7 is installed on an upper surface of the furnace cabinet 6. In addition, an inert gas etc. are supplied to the inside of the furnace core tube 4 from a gas supply port 9a formed in the upper chamber 9. According to this configuration of FIG. 5A, the quartz pipe 10 having the same diameter is downwardly moved together as the glass preform 1 is downwardly moved, with the result that space (capacity) in the drawing furnace can be maintained constant.
On the other hand, for example, Patent Reference 2 discloses a known method for continuously sealing a joining portion of a dummy rod and a tapered portion of a glass preform with a diameter changing greatly without using an upper chamber.
FIG. 5B is a diagram schematically showing a drawing furnace disclosed in Patent Reference 2 described above. This drawing furnace includes a first seal unit 7 with which a gap between a glass preform 1 and an insertion port into a heating furnace is sealed, and a second seal unit 8 for forming a seal so as to cover a taper part 1b when the taper part 1b with a contracted diameter of the glass preform 1 passes through the insertion port.
Like the example of FIG. 5A, the glass preform 1 has a diameter contracted part 1c whose diameter is contracted through the taper part 1b above a straight trunk part 1a, and is hung and supported by joining a dummy rod 2 to the diameter contracted part 1c using, for example, a joining member 3. The heating furnace is constructed by providing the outside of a furnace core tube 4 with a heater 5 and covering the outside of the heater 5 with a heat insulating material and surrounding the whole by a furnace cabinet 6.
An upper surface of the furnace cabinet 6 is provided with the first seal unit 7 with which the straight trunk part 1a of the glass preform 1 is sealed. And, the upper end side of the glass preform 1 is provided with the second seal unit made of a cap member 8 (cylindrical member) having a seal part 8a for sealing the dummy rod 2 insertably so as to surround the taper part 1b, the diameter contracted part 1c and the joining member 3. In addition, an inert gas etc. are supplied to the inside of the furnace core tube 4 from a gas supply port 7a formed in the first seal unit 7.
When drawing of the glass preform 1 progresses and the taper part 1b of the glass preform 1 reaches the first seal unit 7, the cap member 8 which is the second seal unit abuts on the first seal unit 7 as shown by a broken line, and the portions of the joining member 3 and the diameter contracted part 1c above the taper part 1b are sealed. As a result, after the taper part 1b of the glass preform 1 passes through the first seal unit 7, the drawing can subsequently be continued with a sealed state maintained.
In addition, other seal units include, for example, a structure of forming a seal using a pressing mechanism for bringing plural blade members into contact with an outer peripheral surface as disclosed in Patent Reference 3.